Sagittarius

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Sagittarius is a large constellation in the southern hemisphere. Its name means "archer" in Latin. The constellation has its roots in Sumerian culture, but is traditionally depicted as a centaur, half-man, half-horse found in Greek myths. The centaur is depicted wearing a cloak and drawing his bow, aiming an arrow at the heart of Scorpius, the star [7516] Antares. Eratosthenes saw the constellation not as a centaur, but as Crotus, son of the nature god Pan and Eupheme, the nurse to the Muses. Crotus had two legs, not four, and the tail of a satyr. He is said to have invented archery. He lived in the company of the Muses, who asked Zeus to place him in the sky.

Sagittarius is sometimes also identified with the centaur Chiron, son of the titan Cronos and the nymph Philyra. Chiron mentored many Greek heroes: Ajax, Aeneas, Theseus, Achilles, Jason, Heracles and the healer Asclepius among others. He is, however, more commonly associated with the constellation Centaurus.

Ancient Babylonians associated Sagittarius with the god Pabilsaĝ, who had two heads (one panther, one human), wings and was sometimes called "the wild bull with multicoloured legs."

The constellation Sagittarius occupies an area of 867 square degrees and contains 16 stars with known planets. It can be seen at latitudes between +55° and -90° and is best visible at 9 p.m. during the month of August. Sagittarius lies between the constellations Ophiuchus and Capricornus. It is the constellation that contains the greatest number of stars with known planets. The Sun passes through Sagittarius between December 16 and January 14 in sidereal astrology and from November 23 to December 21 in tropical astrology.

[7300] delta Sagittarii is a class K giant with three faint dwarfs for companion stars. [7299] epsilon Sagittarii is the brightest star in Sagittarius and the 36th brightest star in the sky. It is a bright giant 145 light-years distant, almost four times as massive and 375 times more luminous than the Sun. [7301] lambda Sagittarii, an orange giant, marks the top of the lid of the celestial Teapot. It is 77 light-years distant.

[7299] zeta Sagittarii, also known as Ascella ("armpit"), is the third brightest star in the constellation. It is a double star about 90 light-years distant. Its components are a white giant and a subgiant.

[7302] pi Sagittarii, also known as Albaldah ("the axe"), is a triple star approximately 440 light-years distant.

[7304] eta Sagittarii, also known as Sephdar ("violent warrior") or Ira Furoris ("fierce wrath"), is a multiple star system about 149 light-years distant. The primary component is a red giant classified as an irregular variable star. Eta Sagittarii was once referred to as beta Telescopii.

[7323]-[7303] gamma Sagittarii, an orange giant, marks the tip of the Sagittarius’ arrow. It is also sometimes called Alnasl or Nushaba, both names meaning "arrowhead." It is actually a binary system, lying approximately 95 light-years from Earth.

[7298] sigma Sagittarii is the second brightest star in Sagittarius. It is a class B dwarf, about 225 light-years distant. The star is also known as Nunki, a name Babylonians used for the stars they believed represented Eridu, their sacred city. Sigma Sagittarii spins 100 times faster than the Sun, is 630 times more luminous and about seven times as massive. It will not likely live longer than 50 million years.

[7306] tau Sagittarii is an orange giant, about 120 light-years distant. It is also known as Hecatebolus, one of the names of the god Apollo, meaning "sharp-shooter" or "far darter." The star is also notable for being the closest one to the origin of the Wow! signal, a strong narrowband radio signal picked up by SETI scientists in 1977 that may have been a sign of extraterrestrial intelligence.

[7311] alpha and [7312]-[7314] beta Sagittarii are not among the brightest stars in the constellation. Both have a visual magnitude of 3.96. [7311] alpha Sagittarii is a blue dwarf that emits excess infrared radiation and may have a debris disk. It is also known as Alrami and Rukbat. Both names are derived from the Arabic phrase "rukbat al-rami," meaning "knee of the archer."

[7312]-[7314] beta Sagittarii is a binary star, 378 light-years distant. It is composed of two star systems. The star’s traditional name is Arkab ("hamstring"). [7312] beta-1 Sagittarii, or Arkab Prior, is itself a binary star composed of two dwarfs. [7314] beta-2 Sagittarii, or Arkab Posterior, is a white giant 137 light-years distant.

Another notable star that can be seen in Sagittarius is the Pistol Star, a bright blue hypergiant that is one of the most luminous stars discovered in the Milky Way. The Pistol Star is classified as a variable star. It is located in the Pistol Nebula and hidden from view to the naked eye by the interstellar dust that surrounds it. The Pistol Star is part of the Quintuplet Cluster, a dense region full of massive young stars located near the Galactic Centre.

Sagittarius also contains a number of interesting deep sky objects. The Milky Way appears densest in the constellation because that is where the galactic centre is located. The centre is marked by Sagittarius A, a radio-emitting source hidden from view by clouds of dust. Sagittarius A consists of a supernova remnant (Sagittarius A East), a spiral structure (Sagittarius A West), and a bright radio source (Sagittarius A*), located at the core of the spiral. Sagittarius A* is also the top candidate for the location of the supermassive black hole at the centre of the Milky Way.

One of the brightest star clusters in the region is Messier 55, or NGC 6809, located near [7300] delta Sagittarii. Discovered by Nicolas Louis de Lacaille in the 18th century, Messier 55 is a large globular star cluster approximately 17,300 light-years distant.

The Lagoon Nebula, or Messier 8 (NGC 6523), is a large interstellar cloud, 110 by 50 light-years in dimension, identified as an emission nebula. It lies near [7301] lambda Sagittarii and is about 4,100 light-years distant. It is one of the only two star-forming nebulae that can be seen by the naked eye.

The Omega Nebula, or Messier 17 (NGC 6618), is a star-forming region with about 35 hot, bright, young stars radiating energy that produces the reddish glow in the nebula. The stars are hidden in the nebulosity and obscured from view. The Omega Nebula is also sometimes known as the Swan Nebula, the Horseshoe Nebula and the Lobster Nebula.

The Trifid Nebula, or Messier 20 (NGC 6514), is another region of star formation in Sagittarius. It contains a reddish emission nebula, a blue reflection nebula, a dark nebula, and an open cluster of stars. The nebula’s name means "divided into three lobes." NASA’s Spitzer Space Telescope discovered 30 embryonic stars and 120 newborn stars in the nebula in 2005.

The Sagittarius Dwarf Elliptical Galaxy is a loop-shaped galaxy and one of the closest satellite galaxies of the Milky Way. It contains at least four globular star clusters. The one that lies at its centre, Messier 54 (NGC 6715), is approximately 87,000 light-years distant. It was the first globular cluster ever discovered outside the Milky Way. It can be found near [7299] zeta Sagittarii.

The Sagittarius Dwarf Irregular Galaxy, approximately 3.4 million light-years away, is the most distant object believed to be a member of the Local Group of galaxies. It has an apparent magnitude of 15.5.